Teaser-
As in all things poker related...it depends. First you should decide what is important to you (i.e. number of games, easy games, tournamanets, software, cashouts, promotions, etc.). Then you can choose a room.
I've tried probably a half dozen of these places and Party Poker is the best balance of all things I need in a room. With that said, if Party closed tomorrow I would probably head to Pokerstars (assuming that I was still playing online).